At present, a run-flat tire having a side reinforcing layer (a reinforcing layer) with high strength arranged at an inside of a sidewall portion is practically used. The tire keeps rigidity even if it is in a state (inner pressure is zero) in which an air pressure is lost by a puncture, and damage of a rubber is reduced even if it is subject to repeated bending, thus enabling a vehicle to run a certain distance. Thereby, there is no necessity of always having a spare tire ready and weight saving in the whole vehicle can be expected. However, there are limits in a speed and a distance in running a run-flat tire at its puncture. Therefore, a further improvement of durability of the run-flat tire (run-flat durability) is desired.
As a method of improving durability of the run-flat tire, a method of suppressing deformation by thickening a reinforcing layer and preventing fracture by the deformation is mentioned. However, since the weight of the tire is increased, the method runs contrary to the weight saving which is the initial purpose of the run-flat tire.
Further, there is also a method of increasing amounts of reinforcing fillers such as carbon black and enhancing hardness of a rubber for reinforcement to suppress deformation. However, load on a kneader at steps such as kneading and extrusion is increased and since heat build-up of a rubber composition after vulcanization is increased, an improvement of the run-flat durability cannot be expected.
In JP2005-280534A, there is disclosed a run-flat tire in which vibration of the tire is reduced by making the tire have a reinforced rubber layer prepared by a specific method, and the run-flat performance is improved by using a rubber composition containing specified amounts of a specific rubber component, carbon black and silica. However, the run-flat tire is not adequate in the run-flat performance and reduction of tire vibration because it does not contain non-metallic short fibers.